The TME is comprised of cancer and stromal cells, signaling molecules, exosomes, and the ECM, and has long been implicated in the progression, metastasis and resistance to treatment
[8][9]. Cellular and acellular components vary depending on the disease site, for example, the primary tumor site, ascites and secondary tumor site, which modulate the signals received by the cancer cells as summarized in
Figure 1 [10].
Figure 1. Schematic representation of the main cellular components and extracellular matrix composition of the ovarian tumor microenvironment depending on the disease site. (A) In the primary tumor, cancer cells recruit tumor-associated macrophages, cancer-associated fibroblasts, T-cells and endothelial cells. Many extracellular matrix components, such as fibronectin, hyaluronan, tenascin, versican, matrix metalloproteinases (MMPs) and lysyl oxidase (LOX) are upregulated. Collagen progressively remodels into thick fibrils and is randomly oriented. Laminin and collagen IV are underexpressed. (B) Detached single cells or spheroids are immersed in the ascitic fluid, which contains macrophages, fibroblasts, mesothelial cells and immune cells. Extracellular matrix components are found within the aggregated cells and the ascitic fluid. (C) Cancer cells settle onto the mesothelial lining to form secondary tumors that are rich in collagen. Laminin and collagen IV are overexpressed to promote metastasis. Levels of collagen I and III start to decrease.
2.1. Cellular Composition
OvCa cells are a diverse mix of cells with distinct properties and functions. Cancer stem cells (CSCs) represent a small sub-population that have properties of self-renewal, multi-lineage differentiation and resistance to anoikis
[11]. CSCs contribute to tumor initiation, metastasis and resistance to treatment
[12][13]. Several mechanisms within CSCs confer a survival advantage during treatment, including increased resistance to apoptosis, dormancy, the expression of ATP-binding cassette (ABC) transporters, the upregulation of aldehyde dehydrogenases (ALDHs), the response to DNA damage and epithelial-to-mesenchymal transition (EMT). As a result, CSCs lead to disease relapse by escaping treatment and repopulating the tumor with a heterogeneous and more aggressive population of cancer cells
[14].
Within the stromal microenvironment, tumor-associated macrophages (TAMs) constitute the main population of immune cells in the primary tumor and ascites
[15]. TAMs are not only key players in the implantation of cancer cells in the omentum, but also facilitate angiogenesis, metastasis and chemoresistance
[15][16][17]. TAMs, together with myeloid-derived suppressor cells (MDSCs), contribute to the immune escape of cancer cells by hindering the cytotoxic activity of natural killer cells and cytotoxic T cells, inhibiting the maturation of dendritic cells (DCs) and recruiting regulatory T cells
[15][18][19][20]. On the other hand, OvCa-derived exosomes can induce the polarization of macrophages in an M2-like phenotype and the apoptosis of DCs and lymphocytes
[21].
OvCa cells reprogram stromal cells into a pro-tumoral phenotype, like for example, cancer-associated fibroblasts (CAFs). CAFs are reprogrammed fibroblasts that are crucial in the deposition and remodeling of the ECM through the secretion of MMPs and other secreted factors
[22][23]. CAFs secrete pro-inflammatory cytokines (e.g., COX-2, CXL1, CCL5, CXC11 and IL-6) that induce EMT, cancer cell proliferation, invasion, chemoresistance and inhibit cancer cell apoptosis
[22][24][25].
Mesenchymal stem cells (MSCs) can be converted to cancer-associated MSCs (CA-MSCs) and later differentiate into fibroblasts, osteocytes or adipocytes
[26]. The upregulation of TGF-β in CA-MSCs increases the number of CSCs and chemoresistance
[27][28]. Particularly, adipocytes located in the omentum attract cancer cells through the secretion of IL-8 and support cancer cell proliferation through fatty acids
[29][30]. Adipocytes also secrete factors that enhance chemoresistance by activating the Akt pathway
[30]. Elevated levels of VEGF secreted by tumor cells, TAMs, CAFs and adipocytes lead to angiogenesis, endothelial cell survival, proliferation, migration and vascular permeability
[31].
2.2. Matrix Composition
One major constituent of the ovarian TME is the ECM, a meshwork of proteins (e.g., collagen, fibronectin, tenascin and laminin), glycosaminoglycans (e.g., hyaluronan), proteoglycans (e.g., versican) and remodeling enzymes (e.g., MMP2, MMP9 and LOX). The ECM provides both biomechanical and biochemical support for cancer cell proliferation and metastasis
[32]. Continuous remodeling of the ECM, induced by cancer and stromal cells, changes its mechanical properties, thereby influencing disease progression and chemoresistance
[33]. Additionally, cancer cells experience numerous mechanical stimuli within the ovarian TME, such as shear stress, compressive stress, tensile stress and stress relaxation, which directly affect their behavior and signaling pathways
[34].
In normal ovarian tissue, collagen fibers are thin and long while in tumor stroma they are thicker and shorter fibrils perpendicularly aligned rather than parallel to the tumor boundary
[35]. Elevated collagen deposition and remodeling promote tumor progression and drug resistance
[32]. Collagen type I enhances the migration of multiple cancer cell lines due to increased directionality
[36]. At early stages, collagen types I and III are widely distributed; however, the levels decrease as the disease progresses
[37]. During OvCa progression, high levels of collagen type XI have an important role in cell invasiveness, cell proliferation and tumor formation
[38]. Collagen type IV and laminin are often absent in benign ovarian surfaces and primary ovarian tumors. At later stages, the restoration of collagen type IV and laminin presence promotes the spread of OvCa cells to metastatic sites in the peritoneum
[39][40].
Hyaluronan is a major component of the ECM that has been associated with metastatic progression and poor ovarian cancer outcomes
[16][41]. High levels of hyaluronan were detected in metastatic lesions and primary tumors
[42] and correlated with increased versican levels
[32]. In OvCa, the interaction between hyaluronan and its cell surface receptor, CD44, facilitates cell adhesion, migration, tumor growth and peritoneal dissemination
[41][42][43]. Increased expression of fibronectin was found in metastatic tumors and ascitic fluid. Fibronectin is an indicator of poor prognosis that contributes to the formation, adhesion and disaggregation of OvCa spheroids and promotes tumor migration, invasion and metastasis
[32][44]. The stroma also contains tenascin-C that contributes to tumorigenesis and metastasis
[45]. Tenascin-X is considered as potential biomarker of OvCa as it is highly overexpressed in tumorous tissues
[46]. In addition, the downregulation of decorin and lumican are involved in cancer progression and aggressiveness, respectively
[32]. As ECM-associated components, MMPs, mediate ECM remodeling and tumor development. Particularly, MMP-9 and MMP-2 facilitate cancer cell invasion via the degradation of collagen IV at the basement membrane
[16]. Overexpression of LOX enhances the crosslinking of the ECM proteins resulting in increased matrix stiffness
[32].
As outlined above, the bidirectional interactions between cancer cells and the TME are crucial in disease progression. For this reason, it is critical to understand the functions of cancer cells and how the TME regulates the surrounding stroma to promote disease progression.